A strong upward trend in commodity prices since 2000 appears to be at an end, according to Australian Farm Institute executive director Mick Keogh.

He blames a return to political rhetoric for limits on free trade as responsible for stagnation in prices.

Mr Keogh told Tasmanian grain growers the polarisation of international politics from the US to the UK and Europe appears to be ending 15 years of increasing trade in food and agriculture under lower tariff regimes.

"It's certainly quite an important factor associated with that growth in agricultural trade volumes, as China and other countries opened up their markets," Mr Keogh said.

"Whether you look at Brexit or the influence of Trump, there's a return to nationalism, he's anti-free trade.

"What we've seen is that even though Obama is expressly in favour of the Trans Pacific Partnership (TPP), Hillary Clinton has also said she's opposed to the TPP, so in a sense Trump has dragged Hillary across.

"Certainly at the global level it seems as if the enthusiasm for further freeing up agricultural trade has waned from where it was a few years ago."

Mr Keogh said Australian primary producers were also facing increased competition in established markets for their food.

He said Australian dairy and wheat exports were current examples, but red meat was also at risk.

"We're going to be pretty expensive in those global markets," Mr Keogh said.

"We've seen Brazil get access to some of those Asian markets over time.

"We've certainly seen Argentina and Uruguay take over our role in supplying beef into China.

"That really does signal that our competitiveness at these prices is starting to come under real pressure

"So we certainly shouldn't sit back and be complacent and think, you know we've got 700 cents Eastern Indicator here [for beef], we're in clover here, because we're certainly going to get a reaction from the market at that sort of price level."